So it's been explained to me that it's basically just a matter of burning more energy than you consume.

So say, in theory, if I were to do heaps and heaps and heaps of exercise every day. But then I was to also eat takeaway for every meal, but the exercise I was doing burnt more calories than the food I was consuming.

Would that be okay? Would that constitute a healthy lifestyle? Are there any negative consequences of eating improperly that can't be negated with exercise?

Even if you could - which you can't, see below - it would require that you spend hours a day exercising. Look at the calories you burn by doing 20mn of rowing, and see how little it is compared to your daily food intake. Plus, so much exercise is probably unhealthy.
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JDelageJun 11 '11 at 13:59

^^ exactly. Too many people -- myself included at one time -- thought you could eat a double cheeseburger with fries and then burn it all off by running/walking a few miles. The shocker is that you'll probably only burn off around 300 calories which probably only accounts for 1/5th of the total calories consumed in that meal.
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oscilatingcretinJan 18 at 18:20

4 Answers
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There's a video on YouTube called "Sugar: The Bitter Truth", a lecture by Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology, that (from my understanding) implies that, at least in the context of taking in copious amounts of unhealthy sugars such as sucrose, fructose, and high fructose corn syrup, you can offset the bad effects of those sugars by burning them off as quickly as they are processed in the liver. It would definitely take a lot of work to do that though.

However, there are a few problems with the theory of just simply eating junk food. Junk food rarely contains enough useful vitamins, nutrients, and electrolytes to maintain healthy body function, without having to rebuild muscle tissue that is damaged during a workout. Additionally, according to About.com and several other sources, excessive exercise increases your risk of injury.

Most junk food contains high fructose corn syrup, which inhibits the body's fullness mechanism, causing you to eat more than you actually need. This will cause you to need to exercise even more to burn off the excess. It goes without saying that you need proper electrolyte levels. You also need protein, and protein causes you to need calcium, etc., etc. In order for your body to properly process one nutrient, you typically need an appropriate level of two others.

Sodium, potassium, calcium, and magnesium, along with bicarbonate (which your body builds from water you drink and the air you breathe) are important electrolytes. While you're likely going to get plenty of sodium from junk food (probably enough to cause water retention problems), you are unlikely to get much potassium and magnesium, and you aren't likely to be drinking much water.

So, since maintaining an unhealthy diet of junk food will prevent you from taking in the vitamins and nutrients needed to restore damaged tissue, and excessive exercise is a known factor leading to an increase in injury, you're likely to spend more time recovering from extreme muscle soreness or serious injuries than you are going to spend exercising, and thus you won't be burning off the excess unhealthy sugars in your junk food diet.

In the end, you're going to be dealing with a host of short- and long-term health problems by eating unhealthy and thinking you can work it off.

A prime example is diet soda. Diet soda has 0 calories, so theoretically, you could drink infinite portions of the stuff and not gain any weight. However, the body does react to the sweeteners in the soda (even 0 calorie sweeteners) in a tumultuous manner. Within 20 minutes of consuming a sweetened beverage or food, your blood sugar spikes causing an insulin burst. Your liver responds to this by turning any sugar into fat.

While I guess you could loose weight be drinking diet soda and exercising a lot, your body fat will increase which will only lead to wrong results.

So, YES, you might be able to lose weight simply by doing some arithmetic on calories, but NO it does not constitute a healthy lifestyle, and YES there are plenty of negative consequences.

Unfortunately, not all calories are the same. The problem is that your body needs vitamins, minerals, and protein. Due to the way fast food, and most takeout is prepared, the nutritional value is very limited.

You will be at risk of malnutrition for sure if that was your only diet. Even if you supplemented with a multivitamin, you will be hitting your body with the types of foods that increase risk of heart disease (LDL cholesterol), diabetes or insulin resistance (all those simple carbs and sugars), trans fats, etc.

In short, you will be unhealthy and fat unless you can find takeout places that serve food that is actually healthy for you. Those are few and far between. Take it from me, someone who used to eat out a lot, and peaked at 299lb. It took a lot of work to reverse the worst of the effects, and there's more work to do.

First can you burn all the calories? Theoretically Yes. That's already been established. The problem is the calories are not the only "effects" of a crappy diet.

So you need to do all this extra exercise to burn the excess calories. Doing too much exercise isn't great for you because exercise causes oxidation, but if you were a professional and you offset it with supplements and a high intake of antioxidants (you get a bunch of those from healthy food) there wouldn't be a problem.

So you want to do extra exercise which, as you know, is great for stress relief, maintaining a healthy weight and building muscle and bone strength... and eat foods that cause oxidation! So you want to do an activity that causes unhealthy reactions and eat food that causes unhealthy reactions. The food should counteract the negative effects of the exercise so that both activities become as beneficial as possible.

Diet and exercise go hand in hand. A healthy diet by itself can still lead to excessive weight gain if you take in more calories than you burn. Even if all the foods are "healthy!"

It's also been said here that junk food lacks nutrition. So to get the nutrition your body needs you have to consume more, probably even more than you want to or are willing to in order to get your body what it needs to build and repair. All this extra consuming is analogous to all the "extra exercise" and compare it to other machines...

Imagine if you just let your car run in the driveway whenever you weren’t using it. Your not putting any miles on it but your wearing the engine. Think of your body as a machine. Parts get worn out without care and/or excessive use. You're going to put extra food through your digestive tract and force your body process all the crap through your liver kidneys stomach intestines colon etc. Your forcing those organs to do more work for less reward. Do you think they'll last just as long as someone's organs that eats highly nutritious food?